Strain gauge



Aug. 9,1927. I 1 1,638,425

H. L. WHlTTEMO RE STRAIN GAUGE Filed Dec. 10, 1925 NTOR -A TTORNEY Patented Aug. 9, 1927.

UNITED STATES" "PATENT OFFICE.

HERBERT L. WHIT'IEMOBE, O]? WASHINGTbN, DISTRICT OF COLUMBIA.

STRAIN GAUGE.

Applicatlon'flled December 10, 1925. Serial No..'74,516.

\ My invention relates to strain gauges for measuring the elongation or contraction of a gauge-length on the surface of a structural member such as a wall, test specimen,

column or beam, and thereby measuring indirectly, the stress in the member. I An object of my invention is to provide an easily portable strain gauge which may be used to measure accurately small changes in the gauge-length. Further objects of my invention are to provide a gauge which is rugged in construction and with which readings may be easily and quickly obtained. In the accompanying drawings, Figure 1 is a plan View of the gaugewith its pins 10 removed, Figure 2 is a side elevation of the assembled gau efFigure 3 is a sectional elevation along t e line 1-1, and Figure 4 is a perspective view of the gauge.

The main frame of the gauge consists of the two bars 1 and 2, preferably of invar steel which does not change in length when its temperature chan es, and the two th1n flexible lates 3 which are preferably of 2 tempered spring steel, and are fastened to "the bars 1 and 2 by caps 5 and screws 4;

The pin clamps, 6 and 7 are screwed to the bars 2 and 1 respectively, each by means of screws 8. A pin 10 is rotatably mounted in each of the pin-clamps 6 and 7, and may be clamped rigldly by means of the screw 9. Each of the pins 10 has aconical oint 11, adapted to fit into a gauge hole in t e structural member, the point 11 being preferably very slightly offset from the axis of I the pin, as shown in Figure-3. I

By a rotary movement of one. or-both of the pins 10, the distance between points 11 may be changed slightly, thus admitting of some adjustment of the distance between points 11, or gauge-length. The knobs 12 at the tops of the pins 10 may be ressed by the operator to seat the pin mts 11 in gauge holes in the member to e tested.

13 represents a dialmicrometer of a. common. type, provided'with a dial 14, pointer 15, sleeve 18, and operating s indle 19 normally extended from but yiel ingly movable into' the sleeve 18 and dial micrometer 13.

The dial micrometer 13 is firmly bolted through its. downwardly extending lug 16 to the bar 1 by bolts 17.,

The operating spindle '19 abuts with a slight force a amst the abutment block 20 which is rigi y screwed to the bar 2 with very considerable compressive screws 21. A slight motion of the abutment block 20 relative to the dial micrometer 13 causes the operating spindle 19 to move into or out from .the sleeve 18 and dial micrometer 13, and rotates the"pointer 15 proportionally, and in a direction corresponding to the relative motion of the abutment block 20, to the dial micrometer 13. v

The stress in structural members varies from place to place depending upon the load which is supported. As most structural materials follow Hookes law, at least ap roximately, when subjected to external orces, the change in stress at any place on the surface of the member may be measured by observing the change in a gauge-length at that place. The reading of the auge is recorded when there is no load on t e member, then after each load is applied. It is tudinally with relation to-the other bar but prevent other distortion of the gauge. The

relative motion of the pin points ll is transmitted through the pins 10 and the pin" clamps 7 and 6 to the bars 1 and 2 respec- 00 .tively,-which move accurately parallel to each other, for any change in the gauge length within the capacity of theinstrument. 0 The motion of the bar 1 is imparted'to the body of the dial 'micrometer 13 which is 95.. rigidly secured to the bar 1. The motion of the bar 2 is imparted to. the spindle 19 of the dial micrometer 1 3,through the abutment block 20 attached to the bar, 2. Thus,

the relative motion of the bars 1 and-'2, which 1 is equal to-the relative motion ofthe pm points 11,- is indicated on the dial 14 of the dialmicrometer 13 by the pointer 15; The chang: in the length of any gauge-line on'the mem r may, therefore, beobtamed.

Due to the arrangement of the thin .plates 3, they are not "called upon to withstand any force, and hence do not become buckled. The rug construction-of the .frame prevents any 'stortion of the gauge except the bending of the two plates 3 and consequent parallel longitudinal movement of the bars 1 and 2.

It has been found that very accurate readings can be obtained quickly even with an inexperienced operator, and that different operators can obtain the same readings.

I claim:

1. A strain gauge comprising a rectangular frame formed of two parallel bars, sprlng means for securing the bars together, a measuring device secured to one of said bars, means on the other bar for operatin said measuring device, and a pin on each ar to engage the member to be tested.

2. A strain-gauge comprising a rectangu lar frame formed of two parallel bars and two plate springs securing said bars together, a measuring device secured to one of said bars and having an operating member, meanson the other bar for engaging the operating member and a pin on each bar to engage the member tp be tested.

3. A strain gauge comprising a frame formed of two bars, elastically deformable members connecting the bars, means for measuring the movement of one bar relative- 1y to the other bar, and means on each bar to engage the member to be tested.

4. A strain gauge comprising a frame formed of two substantially parallel bars, two plate springs which allow longitudinal movement of one bar relative to the other, means for measuring the said movement and means on each bar to engage the member to be tested.

HERBERT L. WHITTEMORE. 

